U.S. patent number 4,539,083 [Application Number 06/634,523] was granted by the patent office on 1985-09-03 for method for preventing degradation in activity of a low hydrogen overvoltage cathode.
This patent grant is currently assigned to Kanegafuchi Kagaku Kogyo Kabushiki Kaisha. Invention is credited to Toshiji Kano, Takamichi Kishi, Yasushi Samejima, Minoru Shiga.
United States Patent |
4,539,083 |
Samejima , et al. |
September 3, 1985 |
Method for preventing degradation in activity of a low hydrogen
overvoltage cathode
Abstract
Disclosed is a method for preventing a low hydrogen overvoltage
cathode from degradation in activity characterized by adding a
reducing agent to a cathode compartment of an electrolytic cell
which electrolysis an aqueous alkali metal halide solution.
According to the invention, no degradation takes place even after
repeated shutdown of operation.
Inventors: |
Samejima; Yasushi (Kakogawa,
JP), Shiga; Minoru (Himeji, JP), Kano;
Toshiji (Kakogawa, JP), Kishi; Takamichi
(Kakogawa, JP) |
Assignee: |
Kanegafuchi Kagaku Kogyo Kabushiki
Kaisha (Osaka, JP)
|
Family
ID: |
15167044 |
Appl.
No.: |
06/634,523 |
Filed: |
July 26, 1984 |
Foreign Application Priority Data
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Jul 26, 1983 [JP] |
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58-136091 |
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Current U.S.
Class: |
205/350; 205/347;
205/518 |
Current CPC
Class: |
C25B
15/00 (20130101); C25B 1/46 (20130101) |
Current International
Class: |
C25B
15/00 (20060101); C25B 1/46 (20060101); C25B
1/00 (20060101); C25B 001/16 (); C25B 001/26 () |
Field of
Search: |
;204/98,128 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Williams; Howard S.
Assistant Examiner: Chapman; Terryence
Attorney, Agent or Firm: Armstrong, Nikaido, Marmelstein
& Kubovcik
Claims
What we claim is:
1. A method of preventing degradation in activity of a low hydrogen
overvoltage cathode in an electrolytic cell in the electrolysis of
an aqueous alkali metal halide, said electrolytic cell being
partitioned by an asbestos diaphragm or an ion exchange membrane
into an anode compartment containing an anolyte and a cathode
compartment containing a catholyte and being equipped with a low
hydrogen overvoltage cathode, said method comprising adding a
reducing agent to said catholyte during shutdown of the cell.
2. The method of claim 1, wherein the reducing agent is selected
from the group consisting of sulfites, phosphites, hypophosphites,
dithionite, pyrosulfites and mixtures thereof.
3. The method of claim 1, wherein the reducing agent is added in an
amount of from 0.01 equivalent to 100 equivalents per square meter
of the effective area of the low hydrogen overvoltage cathode.
4. The method of claim 1, wherein the reducing agent is added to
the cathode compartment, then operation of the cell is shut
down.
5. The method of claim 1, wherein said electrolytic cell is one of
a plurality of electrolytic cells connected in series or in
parallel in the electrolysis of said aqueous alkali metal halide.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an electrolysis process
of an aqueous alkali metal halide solution using an asbestos
diaphragm or an ion exchange membrane, more specifically, to a
process for preventing a low hydrogen overvoltage cathode from
degrading in activity at the time of shutdown of an electrolytic
cell for use in the foregoing electrolysis.
2. Description of Prior Art
As electrolysis processes of an aqueous alkali metal chloride
solution, a mercury electrolysis process and an asbestos diaphragm
process have been practiced on an industrial scale. However, the
former is being switched to the latter because of environmental
pollution. The asbestos diaphragm process, notwithstanding, has
numerous disadvantages including low quality product and great
consumption of energy, so that an ion exchange membrane
electrolysis process has been developed. It is surmised that in the
near future alkali hydroxide will be produced by the asbestos
diaphragm electrolysis process and the ion exchange membrane
electrolysis process in Japan.
The asbestos diaphragm electrolytic cell and the ion exchange
membrane electrolytic cell involve two different systems of
monopolar type and bipolar type, and mild steel has been heretofore
served as a cathode in every type. Hydrogen overvoltage of mild
steel, nontheless, is as high as 0.3 to 0.4 Volt and thus the study
on low hydrogen overvoltage cathodes to save energy cost is being
actively made. For example, a variety of processes including
plating or spraying of nickel or a nickel alloy are proposed by
Japanese Patent Non-examined Publication Nos. 112785/79, 63686/82,
82483/82, 114678/82 and the like.
However, it has been pointed out that when a low hydrogen
overvoltage cathode prepared by those techniques is installed to
the asbestos diaphragm or the ion exchange membrane electrolytic
cell, activity of the cathode deteriorates to thus result in an
increase in hydrogen overvoltage. The phenomenon appears
notoriously when the operation of the specified electrolytic cell
under operation was shut down for reasons of inspection, changing
of the asbestos diaphragm or ion exchange membrane, exchange of
electrodes and so on.
As a rule, the operation of the specified electrolytic cell among a
plurality of electrolytic cells under operation is shut down by the
use of a short-circuit switch. In such a case, to the electrolytic
cell shut down an electric current reverse to the original
electrolytic current begins to flow instantaneously when
short-circuited. Then, a cathode becomes to be an anode and
dissolution of metal occurs. The dissolution of the metal
presumably occurs selectively from high active portions and the
activity before shutdown is no longer expected even when the
operation is resumed. As the result, cell voltage increases.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
method of preventing degradation in activity of a low hydrogen
overvoltage cathode.
Other objects of the present invention together with advantages
thereof will become apparent to those skilled in the art from the
detailed disclosure of the present invention as set forth
hereinbelow.
Through an extensive series of studies by the present inventors on
the problem of degradation in activity of the low hydrogen
overvoltage cathode at the time of shutdown, it has been found out
that the foregoing objects can be achieved by adding a reducing
agent to a cathode compartment of an electrolytic cell at the time
of shutdown, thus the present invention having been completed.
DETAILED DESCRIPTION OF THE INVENTION
The present invention encompasses a method for preventing
degradation in activity of a low hydrogen overvoltage cathode,
which comprises adding a reducing agent at the time of shutdown to
a cathode compartment of an electrolytic cell for use in an aqueous
alkali metal halide solution which is partitioned by an asbestos
diaphragm or an ion exchange membrane into an anode compartment and
a cathode compartment and is equipped with a low hydrogen
overvoltage cathode.
Investigations have yet to be made to fully clarify the mechanism
of the present invention. Presumably the prevention of degradation
in activity of the low hydrogen overvoltage cathode is attributable
to the following mechanism. Explanation will be made as to an
example in which caustic soda is prepared by electrolysing sodium
chloride with a porous nickel metal served as an active
cathode.
When operation is shut down, the following reaction presumably
takes place on the surface of the low hydrogen overvoltage cathode
of the electrolytic cell;
The reaction (1) is one in which hydrogen absorbed on the low
hydrogen overvoltage cathode is oxydized, having no connection with
the dissolution of metal. After the absorbed hydrogen is consumed
by the reaction (1), the reaction (2) takes place to cause the
dissolution of metal to commence. Electric potential of this
reaction is shown by the following equation; ##EQU1## That is,
though reactive electric potential E is variable according to the
concentration of caustic soda and the ion concentration of nickel,
when [OH.sup.- ] is 10 mol/l and [HNiO.sub.2.sup.- ] is 10.sup.-6
mol/l, the reactive electric potential is -0.859 Volt. Accordingly,
the presence of a reducing agent having oxidation-reduction
potential smaller than -0.859 Volt in caustic soda in the cathode
compartment permits oxidation of the reducing agent to occur before
the reaction (2), whereby the dissolution of nickel is prevented to
thus avoid degradation in activity of the cathode.
The reducing agent usable in the present invention may include
inorganic salts such as sulfites, phosphites, hypophosphites,
dithionites and pyrosulfites. These are used singly or in
combination of two or more, but a salt of the same alkali metal as
that of an alkali metal hydroxide produced is preferred to use,
taking into consideration an influence on the quality of the
product.
A method for adding the reducing agent to the cathode compartment
is not limited in particular, various processes being employed. For
the ion exchange membrane electrolysis, addition of the reducing
agent may be made by adding in the form of an aqueous solution or
solid to a storage tank provided in an aqueous alkali metal
hydroxide liquor-circulating line, a water-supply line provided to
the cathode compartment, or any other manners. For the asbestos
diaphragm electrolysis, it is preferred to provide a reducing agent
aqueous solution-supply line to the cathode compartment through
which it is supplied. Addition may be achieved either continuously
or intermittently. The concentration of the reducing agent aqueous
solution, though not specifically limited, should preferably be
lower than that in which the reducing agent does not precipitate
owing to mutual solubility of three-component system i.e., alkali
metal hydroxide-reducing agent-water. In the case of higher than
the foregoing range, the reducing agent precipitates to plug an
adding inlet of the cathode compartment, which makes it impossible
to supply the reducing agent in an amount desired.
An amount of the reducing agent added is variable according to the
type of the electrolytic cells but should preferably be between
0.01 equivalent and 100 equivalents, more preferably between 0.1
equivalent and 50 equivalents per square meter of the effective
area of the low hydrogen overvoltage cathode. In the case of
smaller than 0.01 equivalent, no adequate effects of preventing
degradation in activity are expected, while an amount exceeding 100
equivalents does not lead to an increase in effects, but to
uselessness. The word "equivalent" used in the invention means a
chemical equivalent per mole of the reducing agent required enough
to change anion of the reducing agent added to the form of ion
which is no longer reactive with oxygen in the aqueous solution.
For sulfites and phosphites, one mole corresponds to two
equivalents, and one mole is four equivalents for hypophosphites
and pyrosulfites, and for dithionites one mole is six
equivalents.
The reducing agent may be added before shutdown, simultaneously
therewith, or several minutes or ten and several minutes
thereafter, but, to obtain the best results, should be added
beforehand to the cathode compartment immediately before shutdown
of the electrolytic cell under operating. After addition, operation
is shut down by a short-circuit device and a busbar had better be
cut off as rapidly as possible on either plus or minus side of the
cell. By cutting off of the busbar, a reverse electric current
circuit formed between the shutdown cell and the short-circuit
device is broken to thus impede the reverse electric current.
Moreover, although an electrolysis equipment is generally comprised
to 20 to 200 electrolytic cells electrically connected in series or
in parallel, the present invention is specifically effective to the
case where the specific one or two or more are shut down
individually, though, of course, effective to the shutdown of all
cells. The economical method for prevention of degradation of
cathode activity has never been proposed by the prior arts but has
been attained for the first time by the present invention.
The present invention will be explained in more detail by way of
Examples and a Comparative Example that follow, to which the
invention is in no manner limited.
EXAMPLE 1
A mild steel plate, 90 mm in length, 40 mm in width and 2 mm in
thickness was subjected to chemical plating with nickel in the
thickness of 30 .mu.m. Next, Raney-nickel particles comprising 50
weight % Al, 45 weight % Ni and 5 weight % Ru were dispersed in a
nickel plating bath with which one side of the nickel-plated mild
steel plate was codeposit plated in the thickness of 250 .mu.m. The
codeposit plated mild steel plate thus obtained, having content of
30 weight % Raney-nickel in the plating thin layer, was immersed in
a 20 weight % aqueous caustic soda solution at 50.degree. C. for 2
hours to thus obtain a low hydrogen overvoltage cathode.
From "NAFION 901" cation exchange membrane manufactured and sold by
E. I. Du Pont de Nemours & Company, an expanded titanium thin
plate anode coated with TiO.sub.2 and RuO.sub.2, and the foregoing
low hydrogen overvoltage cathode, a monopolar type eleactrolytic
cell was fabricated, by which sodium chloride was electrolysed.
Operation was carried out while controlling current density to 23.5
A/d m.sup.2, the temperature to 90.degree. C., anolyte NaCl
concentration to 210 g/l and NaOH concentration to 32 weight%.
Current efficiency was 96%, cell voltage was 3.18 Volts and
hydrogen overvoltage was 0.07 Volt.
After the NaOH concentration in the cathode compartment was diluted
to 22 weight %, a 0.5 mol/l aqueous sodium sulfite (Na.sub.2
SO.sub.3) solution was added to the cathode compartment in an
amount of 20 equivalents per square meter of the cathode and
operation was shut down immediately thereafter by short-circuit
device. After 15 minutes, electric power was supplied again and the
NaOH concentration was returned to 32 weight %. Then operation was
continued for one hour and shut down similarly. Thereafter
operation of the cell and shutdown were repeated 20 times
similarly. Current efficiency of 96%, cell voltage of 3.18 Volts
and the cathode hydrogen overvoltage of 0.07 Volt were retained
after above procedure. No degradation in activity of the cathode
resulting from shutdown was observed.
EXAMPLE 2
An experiment was carried out in a similar fashion to that of
Example 1 with an exception that an aqueous sodium dithionite
(Na.sub.2 S.sub.2 O.sub.4) solution was employed in place of an
aqueous sodium sulfite solution. With the NaOH concentration being
kept to 32 weight %, the aqueous solution containing 0.15 mol/l of
sodium dithionite was added to the cathode compartment in an amount
of 5 equivalents per square meter of the cathode, then the
operation was shut down rapidly by the short-circuit device. After
15 minutes, supply of electric power was resumed. Operation was
continued for one hour and then shut down. Thereafter shutdown was
repeated 20 times similarly and followed by operation again. After
resumption, current efficiency was 96%, cell voltage was 3.18 Volts
and hydrogen overvoltage was 0.07 Volt. There was observed no
degradation in activity of the cathode even after the shutdown.
EXAMPLE 3
An experiment was performed in a similar fashion to that of Example
1, excepting that an aqueous sodium sulfite solution was
substituted with an aqueous sodium hypophosphite (Na.sub.2
HPO.sub.2) solution. With the NaOH concentration of 32 weight %
unchanged, the aqueous solution containing 0.25 mol/l of sodium
hypophosphite was added to the cathode compartment in an amount of
10 equivalents per square meter of the cathode, thereafter the
operation was shut down immediately by the short-circuit device.
After 15 minutes, electric power was supplied again and the
operation was continued for one hour, then shut down similarly.
Thereafter the operation was shut down 20 times by the
short-circuit switch in a similar manner but hydrogen overvoltage
was 0.07 Volt, which showed no degradation in activity of the
cathode.
COMPARATIVE EXAMPLE 1
An experiment was conducted similarly to Example 1, excepting that
a reducing agent was not added. Current efficiency was 96%, cell
voltage was 3.33 Volts and hydrogen overvoltage of the cathode was
0.22 Volt. The results showed degradation in activity of the
cathode caused by shutdown took place.
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